With a need to incorporate low dielectric constant (low-k) interlayer dielectrics (ILDs) into semiconductor technology, has come the realization that low-k ILDs may not seamlessly integrate into existing semiconductor process flows. One place where this is evident is at the back-end saw singulation process. This is because low-k materials are mechanically weaker than conventional silicon dioxide. Consequently, sawing can damage the low-k ILD and adjacent circuitry and impact the yield and reliability of the semiconductor device. To overcome this, some manufacturers use lasers to first scribe through the various layers formed over the semiconductor wafer (including the low-k ILD) and then use the saw to cut through the bulk of the semiconductor wafer, thereby singulating the wafer with a two-step process.
The use of lasers, however, is not without its own set of manufacturing issues. For example, as the laser scribes the wafer it can produce a cloud of debris, and particles from the debris can deposit on exposed wafer surfaces. An illustration of this is shown in the cross-sectional view of the semiconductor wafer 100 in FIG. 1. Unless removed, the debris can impact the yield and reliability of singulated semiconductor devices during/after subsequent packaging.
As shown in FIG. 1, a laser beam 108 ablates and thereby removes from the scribe line region 110 the various device layers 104 formed over semiconductor substrate 102. As a result, debris 112 generated by the laser can deposit onto bumps 106. The debris 112 is problematic because to the extent it is not removed, it can produce non-wetting of the solder bumps during the chip-attach operation. This can result in electrical opens between semiconductor devices and their corresponding packaging substrates.
Shown in FIG. 2A is a cross-sectional view of a semiconductor wafer 200A that has been prepared for laser scribe. Here, the debris 112 problem (shown in FIG. 1) has been addressed by depositing a wafer coat layer 207 over the bumps 206 before scribing the wafer 200A (for the purpose of this specification, a wafer coat layer is a layer formed over bumps on a wafer, so as to protect them during the laser scribe operation). So, as shown in FIG. 2B, instead of depositing onto the bumps 206, the laser-generated debris 212 deposits on the wafer coat layer 207 where it can be rinsed-off later during the wafer coat layer removal process. As shown in FIG. 3, after the wafer coat layer and debris 212 (shown in FIG. 2B) have been removed, a scribe line 210 has been formed through the various device layers 204 of the semiconductor substrate 300 along the wafer's street regions. The scribe line exposes the underlying bulk semiconductor substrate 202 and the bumps 206 have minimal residual debris on them.
However, the wafer coat layer 207 too present a number of integration challenges. For example, the added wafer coat layer increases manufacturing cycle time and can cause materials interactions, which in turn can result in increased oxidation of the die bumps and affect die attach interconnectivity. Also, to the extent that the wafer coat layer is transparent to the laser, it must be removed via some other mechanism besides ablation. And, to the extent that the wafer coat layer 207 optically refracts, diffracts, and/or scatter the laser's beam, it can interfere with the laser's ability to ablate the various underlying device layers.
It will be appreciated that for simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the drawings to indicate corresponding or analogous elements.